Common extension ladders are retractable ladders. Two extension ladders are disclosed in Chinese Patent Nos. 95117319.7 and 98238755.5, and firm and tough ladders, such as firefighter-type ladders made of glass fiber reinforced plastic material, are disclosed in Chinese Patent No. 94230386.5. However, the ladders in the prior art still cannot meet such requirements as simple structure, firm, secure, retractable and easily-adjustable. Moreover, extension ladders in the prior art urgently need to be provided with a simple and reliable safety device to prevent tripping and slipping of such ladders so as to ensure the safety of an individual on such extension ladders.